1. Field of the Invention
This invention relates to buffers. In particular, the invention relates to slew rate control.
2. Description of Related Art
Output buffers in high-speed circuits such as double data rate (DDR) memory and front side bus (FSB) interfaces are important in maintaining data integrity. The signals that are transmitted from the buffer to the external devices outside the device package should have sufficient drive strength and satisfactory slew rate. Slew rate is the rate of change of the signal amplitude. It is typically expressed as the change in voltage with respect to time.
Output pads for a device are subject to ground bounce noise phenomenon. This phenomenon is typically caused by the switching of voltage from one logic level to another. When several outputs change state simultaneously, the combined current variations at these outputs generate noise on power rails which has a significant affect on the rate of change of the signal amplitude. Slew rate control becomes important when there are many outputs switching at the same time, or when the data rate is high.
Therefore, there is a need to have an efficient technique to improve slew rate performance.